Looks like you're confusing Mb/s (megabit per second) with MB/s (megabyte per second).
1000 Mb/s becomes a theoretical 125 MB/s, and 120 MB/s looks like good performance (since you don't give more information, I take that it is a standard desktop PC with SATA hard disks).
Besides, I don't really think you can reach 1 GB/s (which would mean 8 Gb/s) without special equipment (10 Gb ethernet, a high-end NAS or a SAN etc...).
